The present invention is directed to an opaque, low resistivity silicon carbide. More specifically, the present invention is directed to an opaque, low resistivity silicon carbide that is opaque within a specific wavelength of light.
Silicon carbide, especially silicon carbide produced by chemical vapor deposition (CVD-SiC), has unique properties that make it a material choice in many high temperature applications. Chemical vapor deposition processes for producing free-standing silicon carbide articles involve a reaction of vaporized or gaseous chemical precursors in the vicinity of a substrate to result in silicon carbide depositing on the substrate. The deposition reaction is continued until the deposit reaches the desired thickness. To be free standing, the silicon carbide is deposited to a thickness of upward of about 0.1 mm. The deposit is then separated from the substrate as a free-standing article that may or may not be further processed by shaping, machining, or polishing and the like to provide a final silicon carbide article.
In a chemical vapor deposition silicon carbide production run, a silicon carbide precursor gas, such as a mixture of methyltrichlorosilane (MTS), hydrogen and argon, is fed to a deposition chamber where the mixture is heated to a temperature at which the mixture reacts to produce silicon carbide. Hydrogen scavenges chlorine that is released from the MTS when the MTS dissociates during the reaction. An inert, non-reactive gas such as argon or helium is employed as a carrier gas for MTS (a liquid at room temperature). Inert gases also act as a diluent whose flow rate can be varied to optimize the reaction and assures removal of by products from the reaction/deposition zone. The silicon carbide deposits as a layer or shell on a solid mandrel provided in the deposition chamber. After the desired thickness of silicon carbide is deposited on the mandrel, the coated mandrel is removed from the deposition chamber and the deposit separated therefrom. The monolithic free-standing article may then be machined to a desired shape. Several CVD-SiC deposition systems are described in U.S. Pat. Nos. 5,071,596; 5,354,580; and 5,374,412.
Pure CVD-SiC has relatively high electrical resistivity. While this is a desirable characteristic for certain applications, such a characteristic is a limitation restricting its use in other applications. Certain components, such as plasma screens, focus rings and edge rings used in plasma etching chambers need to be electrically conductive as well as possess high temperature stability. While high temperature properties of CVD-SiC have made it a material of choice for use in such chambers, its high resistivity has limited its use in fabricating components that require a greater degree of electrical conductivity.
High electrical resistivity of CVD-SiC has further restricted its use in applications that are subject to the buildup of static electricity. The need to ground components used in such applications requires that they possess greater electrical conductivity than is generally found in CVD-SiC. A low resistivity silicon carbide would provide a unique and useful combination of high temperature properties with suitable electrical conductivity properties for use in applications where grounding is required.
U.S. patent application Ser. No. 09/790,442, filed Feb. 21, 2001, (non-provisional of provisionally filed U.S. application No. 60/184,766, filed Feb. 24, 2000), assigned to the assignee of the present application discloses a chemical vapor deposited low resistivity silicon carbide (CVD-LRSiC) and method of making the same. Electrical resistivity of the silicon carbide is 0.9 ohm-cm or less. In contrast, the electrical resistivity of relatively pure silicon carbide, prior to the CVD-LRSiC of the application Ser. No. 09/790,442, is in excess of 1000 ohm-cm. The method of preparing the CVD-LRSiC employs many of the same components as the CVD methods disclosed above except that nitrogen is also employed. The lower resistivity of the silicon carbide is believed to be attributable to a controlled amount of nitrogen throughout the silicon carbide as it is deposited by CVD. The nitrogen is incorporated in the deposit by providing a controlled amount of nitrogen with the precursor gas in the gaseous mixture that is fed to the reaction zone adjacent a substrate. The reaction is carried out in an argon gas atmosphere. As the silicon carbide precursor reacts to form the silicon carbide deposit, nitrogen from the gaseous mixture is incorporated into the deposit. The CVD-LRSiC contains at least 6.3×1018 atoms of nitrogen per cubic centimeter of CVD-LRSiC.
While the resistivity of CVD-SiC can theoretically be lowered to a desired level by the introduction of a sufficient amount of impurities, the resulting elevated levels of impurities adversely affect other properties of the SiC such as thermal conductivity and/or high temperature stability. The CVD-LRSiC is relatively free of impurities, containing less than 10 ppmw of impurity trace elements as determined by gas discharge mass spectroscopy. The CVD-LRSiC is further characterized by thermal conductivity of at least 195 Watts/meter degree Kelvin (W/mK) and a flexural strength of at least 390 MPa.
The CVD-LRSiC is electrically conductive and possesses high temperature stability in addition to being a high purity SiC. Thus, the free standing CVD-LRSiC may be readily employed in high temperature furnaces such as semiconductor processing furnaces and plasma etching apparatus. The CVD-LRSiC may be sold as a bulk material or may be further processed by shaping, machining, polishing and the like to provide a more finished free-standing article. For example, the CVD-LRSiC may be machined into plasma screens, focus rings and susceptors or edge rings for semi-conductor wafer processing and other types of high temperature processing chamber furniture as well as other articles where CVD-LRSiC material is highly desirable.
In the manufacture of semi-conductor wafers, there are numerous process steps. One set of steps is referred to as epitaxial deposition, and generally consists of depositing a thin layer (between about 10 to less than one micron) of epitaxial silicon upon the wafer. This is achieved using specialized equipment such as SiC wafer boats or SiC susceptors or edge rings to secure the semi-conductor wafers in processing chambers, and a chemical vapor deposition (CVD) process. The CVD process requires that the wafer be heated to very high temperatures, on the order of 1200° C. (2000° F.).
There has been a recent trend in the semi-conductor art to employ equipment that operates upon a single wafer, rather than a group of wafers. In single wafer equipment the heating of the wafer to the CVD temperature is greatly accelerated such that the wafer is taken from about room temperature to an elevated temperature within about 30 seconds. Such processing is known as rapid thermal processing or RTP. RTP includes depositing various thin films of different materials by an RTP-CVD process, rapid annealing of wafers (RTP thermal processing) and rapid oxidation to form silicon dioxide. While the silicon wafer can accept such rapid temperature change well, the wafer must be held in position by a susceptor or edge ring that can also withstand such rapid temperature changes. Susceptor or edge rings composed of CVD-SiC or CVD-LRSiC have proved very suitable for withstanding RTP conditions.
Many RTP systems employ high intensity W-halogen lamps to heat semi-conductor wafers. Pyrometers are used to measure and to control wafer temperature by controlling the output of the W-halogen lamps. Accurate and repeatable temperature measurements for wafers over a wide range of values are imperative to provide quality wafers that meet the requirements for integrated circuit manufacturing. Accurate temperature measurement requires accurate radiometric measurements of wafer radiation. Background radiation from W-halogen lamps (filament temperature of about 2500° C.) or from other sources can contribute to an erroneous temperature measurement by the pyrometer especially at low temperatures (about 400° C.) where the radiant emission from the wafer is very low compared to the lamp output. Also, any light from the W-halogen lamps that passes through (transmits) a susceptor or edge ring can cause an incorrect temperature reading by the pyrometer.
The industry has addressed the temperature problems by designing the RTP chamber with single sided heating and mounting the pyrometers on the chamber bottom opposite the light source. To further reduce light interference, the area under the wafer was made “light tight”, thus eliminating stray reflected light from entering the area under the wafer. In addition to redesigning the RTP chamber, CVD-SiC or CVD-LRSiC edge rings and susceptors were made opaque to W-halogen lamp light in the wavelength range that pyrometers operate by coating the rings with 200 μm (0.008 inches) of poly-silicon. However, coating edge rings with poly-silicon adds substantial cost to the edge rings. Further, the coating process (epitaxial silicon growth) has many technical problems associated with it such as dendritic growth, bread loafing around edges and purity problems that reduce yields. Poly-silicon coating adds thermal mass to the edge rings. The increased thermal mass limits heating ramp rates during RTP processing cycles. Ideally, edge rings have a thermal mass that is as low as possible to achieve the fastest heating ramp rates. The faster the ramp rate the shorter the processing cycle time for wafers, thus reducing wafer processing costs. Another advantage to faster ramp rates is that the total integrated time at high temperature for the wafers is reduced allowing for less diffusion of any dopant species employed during processing. Such is highly desirable as the feature sizes decrease for semi-conductor devices (trend in the semi-conductor industry). As the feature size gets smaller the distance traveled by dopant atoms also gets smaller.
Accordingly, although there are highly suitable CVD-LRSiC articles that may be employed in semi-conductor wafer processing chambers, there is still a need for improved CVD-LRSiC articles that are opaque at certain wavelengths.